Phase-shift modulation (PSK) and quadrature phase-shift modulation (QPSK) are becoming more important in radio frequency communication systems. This is especially true in satellite communications where the available frequency spectrum is limited and costly.
Prior art phase-modulation (PM) systems have several problems that must be overcome in order to keep pace with the continued demands for effective communication in the pertinent frequency spectrums. One well known type of PM system uses balanced mixers, which require waveform shaping for spectral control. Most waveform shaping techniques result in an uneven amplitude which interferes with class-C power amplification and causes established communication links to be dropped. Consequently, the industry has looked to other PM techniques having improved performance.
One such technique implements PM using strategically placed delay lines which are used to delay the carrier signal and effect the PM based on the voice signal or data signal. Delay lines, unfortunately, require considerable space, and, as the use of integrated circuits continues to increase, the availability of such space will continue to decrease.
Two more popular PM techniques are direct digital synthesis (DDS) and digital signal processing (DSP). DDS allows the carrier signal to be directly modulated but requires relatively large and expensive frequency synthesizer designs. DSP is becoming more widely used with advances in the speed and functionality capabilities of DSP integrated circuits. However, the complexity and cost of DSP would be justified only if it is needed for other reasons.
Another PM approach uses a phase-locked loop (PLL) in the synthesizer portion of the radio transmitter. One technique is exemplified in U.S. Pat. No. 4,313,209 (E. R. Drucker). A disadvantage of his approach is that the injection of the PM input into the PLL affects the loop gain and bandwidth.
Accordingly, there is a need for a PM technique that overcomes the above-mentioned deficiencies.